1. Field of the Invention
The present invention is related to photovoltaic devices such as solar cells. More particularly it relates to metal wrap through solar cells improved by specific passivation scheme, and to a cost effective method of manufacturing the same.
2. Description of the Related Technology
Most solar cells described in the prior art can be subdivided into several categories according to their general structure.
One of these categories is the group of the so called back-contacted solar cells, meaning that both ohmic contacts to the two oppositely doped regions of the solar cells are placed on the back or non-illuminated surface of the solar cell. This concept avoids or reduces shadowing losses caused by the front metal contact grid on standard solar cells.
The most straightforward way to fabricate back contact solar cells is to place the carrier collecting junction between semiconductor regions of opposite doping close to the back surface of the cell (“back-junction” cell). The document “1127.5-Percent Silicon Concentrator Solar Cells” (R. A. Sinton, Y. Kwark, J. Y. Gan, R. M. Swanson, IEEE Electron Device Letters, Vol. ED-7. No. 10, October 1986) describes such a device.
Since the majority of photons are always absorbed close to the front surface of the cell, the generated carriers in these regions have to diffuse through the entire base region of the cell towards the carrier collecting junction close to the back surface. For this concept, high quality material with minority carrier diffusion lengths longer than the cell thickness is needed, which makes this solution not applicable for most solar grade materials which generally have short diffusion lengths. Additionally, a perfect front surface passivation is required for cells having the carrier collecting junction close to the back surface.
The largest group of solar cells has the carrier collecting junction close to its front surface. The current from these solar cells is collected by a metal contact to the doped region on the front surface and by a second contact to the oppositely doped region on the back surface. Although this front grid structure can be optimized relatively easily in order to get high collection efficiencies, the trade off between resistance losses and shading losses necessitates a coverage of the front surface by 6-12% of the total area.
Another group of solar cells combines the two approaches. Such solar cells have both external contacts to the oppositely doped regions on the back surface and the collecting junction close to the front surface. The collected current from the front surface is lead through openings, which extend through the entire wafer, to the back surface. Using this structure, shading losses normally arising from the front metallization grid are greatly reduced.
The document WO 1998/054763 (EP 0 985 233 B1) describes such a structure hereafter called Metal Wrap Through (MWT).
Despite their decreased shadowed area those MWT solar cells are still generally efficiency limited due to excessive shunt like recombination happening in the via and under the rear emitter bus bars (as disclosed e.g. in documents: “Processing and comprehensive characterization of screen-printed MC-Si Metal Wrap Through (MWT) Solar cells”, Clement et al., Proceedings of the 22nd European Photovoltaic Solar Energy Conference, Milan, Italy (2007), p1400-1402; “Lifetime studies on laser drilled vias for application in Emitter-Wrap-Through-Solar Cells”, Mingirulli et al., Proceedings of the 22nd European Photovoltaic Solar Energy Conference, Milan, Italy (2007), p 1415-1418).